When storming the Firefly house, Sheriff Wydell is loading a Remington 1100 shotgun (a semiautomatic), but when he is shown from behind firing upon the house he is clearly using a Remington 870 (a pump action shotgun).
Two of the officers storming the house at the beginning of the movie are firing M-16's, which fire a .223. The armor the Fireflies wore would likely not have provided sufficient protection at the ranges the officers fired from.
The license plate from the vehicle that is stolen from the maid by Baby and Otis is an all numbers license plate. This type of license plate is only used by government vehicles. Civilian vehicles would have both numbers and letters.
When Otis, Baby and Spaulding are riding in the car toward the road block, after the incident with Wydell, there is blood all over the car, except not on the lower part of Baby's leg where she was shot previously.